1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic system such as copying apparatus, printer, facsimile apparatus (FAX), or the like for performing an image exposure by a laser beam and, more particularly, to a correction of an improper scanning position.
2. Description of the Related Art
In an electrophotographic system for performing an image exposure by a laser beam, a method whereby the laser beam is irradiated to a rotary polygon mirror (hereinbelow, referred to as a polygon mirror) and is deflected thereby and a surface on a photosensitive material is exposed by the deflected reflection light is used. As a photosensitive material, it is desirable to use a photosensitive material having such a shape that it is located at an equal distance away from a light source of the laser beam, that is, it draws an arc from a reflecting surface of the polygon mirror. However, in order to form an image after the exposure, a cylindrical photosensitive material is used in many image forming apparatuses.
By using the cylindrical photosensitive material, lengths of optical paths from the respective light sources to the photosensitive material differ, so that a difference occurs in main scanning magnifications by the laser beam. As a method of correcting the difference in the main scanning magnifications, there is a method whereby an optical device such as an fθ lens or the like is provided between the light source and the photosensitive material. However, a high precision is required for the correction that is made by the optical device. In the many image forming apparatuses, therefore, the high-precision correction of the optical device is not performed but the main scanning length is divided into a plurality of areas and the magnification in the main scanning direction is adjusted by an insertion/extraction of small pixels (hereinbelow, referred to as “pixel segments”) every division area.
A clock frequency showing the pixel segments is obtained by a method whereby a clock for transferring image data of one pixel is multiplied by a desired resolution. That is, assuming that the resolution is equal to 16, the clock frequency showing the pixel segments is equal to a frequency which is 16 times as high as the clock frequency for transferring one pixel. If the resolution is equal to 32, the clock frequency is equal to a frequency which is 32 times as high as the clock frequency for transferring one pixel.
The adjustment of the magnification in the main scanning direction is executed by using the foregoing multiplied high-frequency clock as a unit. Since the adjustment by the insertion/extraction of the pixel segments is the adjustment in the main scanning direction, if the adjustment is made to the image data of a line unit at the same position, the pixel positions which are subjected to the adjustment become continuous in the sub-scanning direction.
If the adjustment by the insertion/extraction of the pixel segments is performed to the same position in the main scanning direction, a texture appears at a period when the insertion or extraction is executed and it results in a deterioration in image quality. Therefore, there is an apparatus which makes such control that the insertion/extraction of the pixel segments is not performed at the same position in the main scanning direction by the line unit in which the adjustment is performed. According to Japanese Patent Application Laid-Open No. 2004-098592, by providing a random number control portion and a plurality of counting units, the above control is performed. Specifically speaking, timing for starting the operation of the counting unit serving as a base of calculation of an adjusting position is deviated every input of the line unit, thereby controlling so that the correction by the pixel segment insertion/extraction is not performed to the same position in the main scanning direction.
However, although Japanese Patent Application Laid-Open No. 2004-098592 discloses that the control is made so that the pixel-segment-insertion/extraction is not performed to the same position in the main scanning direction, uniform control is made irrespective of a type of image data to be processed. However, an effect which is obtained as a result of that the pixel segment insertion/extraction positions are not made continuous in the sub-scanning direction depends on the type of image data. For example, although the effect which is obtained since the pixel segment insertion/extraction positions are not continuous in the sub-scanning direction appears to a natural image such as a photograph or the like, in the case of a character or a diagram, the main scanning positions where the pixel segment insertion/extraction is executed become discontinuous in the sub-scanning direction, so that the image quality deteriorates.